05 groundwater contamination assessment in the al-quwy'yia

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precipitation of \100 mm while the annual potential

evapotranspiration exceeds 3,000 mm (Al-Saleh 1992).

The main source of water supply for the whole area is

groundwater either from shallow or deep aquifers

depending on the location and depth of the productive

wells. The groundwater is generally used for domestic

purposes, but there is also lack of knowledge concerning

water quality as in some cases the local Bedouin drink theextracted groundwater directly from the wells.

To highlight this contamination problem and to put in

place low cost method for tracing the pollutants in the

subsurface, geophysical surveys utilizing geoelectrical

techniques have been applied (Asfahani 2007; Yadav and

Singh 2007; Porsani et al. 2012). Time-domain electro-

magnetic and electrical resistivity methods are the pre-

ferred geoelectrical techniques and have wide applications

in delineating the shallow subsurface properties and tracing

pollution plumes (Pellerin 2002; Delgado Rodrı́guez et al.

2012). Both methods measure the contrast in electrical

conductivity properties of the subsurface rock units, whichvary according to rock type, water content, water quality

and temperature (Parasnis 1997). In the same rock units,

electrical conductivity is mainly controlled by the water

content and its quality within the formation matrix rather

than by the solid granular material itself. An increase in ion

dissemination in the groundwater due to pollution has a

direct effect on the groundwater electrical conductivity

properties (Metwaly et al. 2012a). This fact has been well

studied using an empirical relationship of Waxman and

Smits (1968), which suggested that as long as there are free

ions in the groundwater electrical conductivity will

increase. Transient electromagnetic method (TEM) and 2D

electrical resistivity tomography (2D ERT) data sets have

been utilized as fast, low cost and effective ways for

tracing low resistivity zones in the subsurface, which are

coincident with contaminated groundwater. Moreover, in

the present study, the location of the basement rocks, which

Fig. 1 Location map of the

dump site and the different

acquired data sets at Al-

Quwy’yia area. st # is the TEM

stations, v # is the VES station,

QAP2-1 is the borehole in the

study area and Qpro # is the 2D

ERT profiles

Fig. 2 Photos for the Kuff Fm. outcrop (a) and surface bond of the

wasted water at the dump site (b) Al-Quwy’yia area

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is dipping away toward the eastern part of the whole area,

may be traced using the two data sets.

Geological and hydrogeological setting

Until now, few geophysical studies have been carried out

along Al-Quwy’yia environs for the purpose of shallowsubsurface groundwater evaluation (Metwaly et al. 2012b).

Most of the works are on the geological and mineral

resources as the area is situated very close to the eastern

part of the Arabian shield (Senalp and Al-Duaiji 2001). Al-

Quwy’yia area is located close to the Arabian Shield,

which is composed of a basement complex bordering the

western side of the study area (Fig. 1). To the eastern side,

the sedimentary units overlie the basement rocks (Fig. 2a).

At the surface, there are weathering products (alluvium and

eolian deposits), which cover most of the area. Underlying

the weathering sediments, the lithological unit is composed

of Khuff limestone, which overlies the basement rocks(Fig. 2a). The shallow aquifer is exposed at the surface and

hydraulically extends to the Khuff limestone. The shallow

aquifer is recharged from the sparse rainfall over the

basement and limestone exposures (Nebert 1970).

Different types of land-usage are recognized within the

study area. The central part is represented by Al-Quwy’yia

City (urban built-up and rural areas), while the western and

southwestern parts are represented by basement complex

rocks. The Holocene alluvial deposits fill the wadi courses

within the study area. These wadi fill deposits have good

hydraulic characteristics, enhancing the groundwater

recharge as well as infiltrating the waste water originating

from either septic tanks or dump sites. The water table

contour map for the shallow Khuff aquifer is based on

measuring the depth to the water at 22 wells over the whole

Al-Quwy’yia area (Fig. 3). The general trend of ground-water flow is from the west and the southwest towards the

east and the northeast directions. These flow directions are

coincident with regional topographic trends.

Methodology and data acquisition

The time-domain electromagnetic method (TEM) and 2D

electrical resistivity methods (ERT) are well known in

exploration geophysics (Telford et al. 1995; Nabighian and

Macnae 1991). The methods have been applied together in

many ways making them ideal partners for shallowexploration. Although both methods measure the electrical

conductivity or resistivity of the subsurface, they sample

different volumes and have different degrees of sensitivity.

TEM is an inductive technique and has an area of inves-

tigation that is a function of the descending and expanding

image of the transmitted current. This area is typically

40 m 9 40 m or greater. The resistivity method is a gal-

vanic technique that samples a linear portion of the ground

Fig. 3 Water table contour map

of Khuff aquifer in Al-Quwy’yia area (2012)

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as defined by the area of current flow. The TEM method

gives an absolute measurement of the subsurface resistivity

while the electrical resistivity method gives a relative

measure of this quantity (Auken et al. 2001).

In this work, the TEM data sets have been acquired

using the TEM FAST 48HPC system with a single trans-

mitter and receiver loop (coincident loop) of length

50 m 9 50 m (AEMR 2007). The time of measurementranged from 4 to 16 ms including 48 time windows with a

repeatability frequency changing from 3.2 kHz to 11 Hz.

More details about the TEM FAST 48HPC system can be

found in the operating manual. The principles of the

TDEM method are described in Barsukov et al. (2006). 2D

electrical resistivity surveys are commonly used for shal-

low subsurface investigations particularly environmental

surveys (Loke 1999). Because of its efficiency and effec-

tiveness in producing images of the subsurface, the 2D

geoelectrical resistivity imaging actually measures the

apparent resistivity of the subsurface, which can be

inverted to develop a model of the subsurface structure andstratigraphy in terms of its electrical properties (Loke

2003). The resistivity of the subsurface is affected by

porosity, amount of water, ionic concentration of the pore

fluid and composition of the subsurface materials. There-

fore, the resistivity data can be used to identify, delineate

and map subsurface features such as electrically conductive

contamination plumes (Dawson et al. 2002). The acquired

2D data sets in this work have been collected using the

SYSCAL PRO system, which features 72 electrodes with a

dipole–dipole electrode configuration and minimum elec-

trode offset equal to 5 m. Details about the survey and the

2D electrical resistivity method are available elsewhere

(Griffiths and Barker 1993; Loke and Barker 1996). In

addition, vertical electrical sounding (VES) has been con-

ducted close to one of the TEM stations and nearby to the

borehole (QAP2-1) to relate the responses of the geoelec-trical techniques (TEM and VES) to the different subsur-

face lithological units in the study area (Fig. 1). Moreover,

22 water samples were collected and chemically analyzed

during the summer of 2012. The results for nitrate (NO3)

concentration are presented here and serve as a useful

source indicator of anthropogenic groundwater contami-

nation (Fig. 4).

Results

Nitrate concentrations

The nitrate concentrations in 22 boreholes in Al-Quwy’yia

region ranged from 1 to 49 mg/L, with a mean and SD of

37.5 and 14.4 mg/L, respectively (Fig. 4). These results

contrast markedly with that for non-polluted groundwater,

where typical nitrate levels are much less than 1 mg/L.

Thus, the presence of nitrate in groundwaters at levels

greater than about 3 mg/L usually reflects the impact of

Fig. 4 Areal distribution of

nitrate concentrations (mg/L) of the Khuff aquifer

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human activities on well water quality. Nitrate concentra-

tions in groundwater [10 mg/L indicates significant con-

tamination that is usually of biogenic origin (Mueller et al.

1995). Based on the classification of approach of Madisonand Brunett (1985), 45.6 % of the samples in this study had

nitrate concentrations of \44.3 mg/L, while the nitrate

levels for 54.5 % of the samples were [44.3 mg/L

(Table 1). The spatial distribution of nitrate was interpo-

lated using the kriging method and the fitted variogram,

showed that the southeastern part of the study area gener-

ally had the lowest nitrate concentrations (Fig. 4). These

low concentration levels indicate a limited impact of

human activity on the fresh water flow from the western

and southwestern basement rocks. Higher concentrations of

nitrates ([44.3 mg/L) were dominant in the urban regions

and at dump sites. These higher values of nitrate were

related to the infiltration and seepage of sewage from septic

tanks and the dumping of waste water along the south-

eastern part of the urban area (Fig. 4). Thus, the concen-

tration profiles and distribution patterns for NO3 suggests

that the aquifer has been already affected by the infiltrationof pollutant chemicals from the surface.

Comparison of the VES model with the borehole logs

To understand the different responses of subsurface litho-

logical units in the resistivity model; the VES (B1) was

been carried out close to a borehole (QAP2-1) located near

a waste dump site (Fig. 1). In the lithological description

there was no indication of groundwater contamination in

this borehole. The uppermost alluvium has at least two

resistivity layers based on the moisture and lithological

contents of this zone (Fig. 5). It extends to a depth of about

7 m from the ground surface. The limestone layer of the

Khuff Formation also has two resistivity layers on the VES

model based on the clay contents and extends to a depth of

about 96 m. Indeed, it shows relatively low resistivity

values in comparison with the alluvium layer. Most of the

recorded groundwater was been defined in the Khuff

limestone layer. Then the basement complex was been

recorded at a depth of about 96 m and showed a high

resistivity characteristics in the VES model (Fig. 5).

Comparison of VES and TEM data sets

After recording the signatures of different lithological units

in the resistivity models, the VES and TEM data sets were

compared with the resultant models of the two data sets.

VES 13 and TEM 32 are an example of this comparison

(Fig. 6). There is good consistency between the two

inverted models as they represent the same subsurface

lithological units, but the measurements for each technique

were carried out differently. At shallow parts of the VES

model there are three layers, which are represented in the

Table 1 Nitrate concentrations in groundwater of the Khuff aquifer

Zone Nitrate

concentration

Sample

no.

Sample

%

Remarks

I \0.89 mg/L 0 0 Assumed to represent

natural background

concentrations

II 0.89–13.29

mg/L

2 9.1 Transitional;

concentrations that

may or may not

represent human

activities

III 13.29–44.29

mg/L

8 36.4 Indicates elevated

concentrations

resulting from

human activities

IV [44.29 mg/L 12 54.5 Exceeds maximum

concentration for

National Interim

Primary Drinking-

Water Regulations

Fig. 5 Comparison of VES B1

model with the borehole

(QAP2-1) lithological

succession: a the filed curve of

VES B1, b the inverted

resistivity-depth model (1) and

the equivalence models (2) in

comparison with the different

lithological units

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TEM model by only one layer. This relates to the dense

electrode separations at the small offsets in acquiring the

VES data and the late time of recording the TEM mea-

surements. In addition, the VES and TEM data sets have

almost the same response for the possible contaminated

zone of the limestone layer at 45 m depth from the ground

surface and extending to a depth of about 55 m (Fig. 6). It

is possible to trace the variations in the measured TEM data

Fig. 6 Comparison of VES 13

with TEM 32 conducted at the

same site: a the filed curves of

VES 13 and TEM 32,

(b) inverted resistivity-depth

models with the interpreted

lithological units

Fig. 7 Examples of themeasured data sets along the 1st

profile with the corresponding

inverted resistivity models:

a the measured field curves,

b the inverted resistivity-depth

models

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Underneath the surface layer the limestone layer is domi-

nant with relatively high resistivity values (25–80 X m).

However, inside this layer and close to the dump site

(nearby stations 48, 49 and 50 in the 1st profile and 55 in

the 2nd profile) the resistivity values decrease again

because of the effects on the polluted groundwater from the

dump site. Underneath the TEM stations (st. 50, 51 and 54

in the 1st profile and st. 51 and 54 in the 2nd profile) the

resistivity values increase ([150 X m) because of the

basement complex occurrence. On the other hand, there is

no evidence of subsurface contamination from the dump

site for the third profile (Fig. 8c) as indicated by the similar

resistivity values for the limestone rock. This profile is

rather far away from the dump site location.

Results for 2D ERT

2D ERT data were acquired along two profiles close to the

known dump site. The acquired data reflect relatively dense

sampling of the subsurface in comparison with the pro-cessed TEM profiles. To produce cross-sections showing

the subsurface resistivity distribution in a 2D manner,

RES2DINV software utilizing a smoothness constraint was

used. The calculated data were compared with the field

data and the resistivity model was updated based on the

difference between the observed and calculated data. This

procedure was continued until the calculated data matched

the actual measurements with an acceptable level of error.

There were two profiles under consideration with one being

parallel to the dump site, while the other was at the right

angles to the dump site. Inspection of the inverted 2D

electrical resistivity profiles acquired very close to the

dump site (Fig. 1) revealed many important features

(Fig. 9). The parallel profile was more affected by the

sewage water seepage in the subsurface producing a rela-

tively low resistivity for the limestone layer (Fig. 9a). The

thickness of the contaminated layer is more than 60 m and

this reduces in going away from the dump site as the

basement rocks start to be traced (Fig. 9b). The 2D ERT

profiles confirm the results of the TEM data sets and show

that the contaminated zones have low resistivity

characteristics.

Conclusion

The study area of Al-Quwy’yia is considered a promising

region for agricultural and industrial projects, but sustain-

able development of the area is compromised by pollutionof the limited groundwater resources. The rapid growth of

human activities in the study area is accompanied by the

contamination and overexploitation of the groundwater

resources. The study identified possible anthropogenic

processes affecting the groundwater quality of the Khuff

aquifer in the study area. The nature and spatial distribution

of the contamination observed in the aquifer indicated that

the main contamination sources came from seepage of

sewage water from septic tanks and dumping of waste

Fig. 9 2D ERT Profiles acquired close to the dump site: a parallel to the dump site, b at the right angle to the dump site

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water outside of the urban areas. The TEM and 2D ERT

techniques have been applied in an attempt to delineate the

contaminated areas and obtain responses for both data sets.

The measured data at contaminated sites exhibited rela-

tively low resistivity characteristics in comparison with the

unaffected areas. Therefore, as long as there are low

resistivity zones in the measured data sets, this is an indi-

cation of contaminated plumes in the groundwater. Suchphenomena have been confirmed using the calibration

process between the measured data sets and the lithological

units in the study area. The TEM and 2D ERT both gave a

clear indication of the contaminated areas, but at different

spatial resolutions. The TEM technique resolved the con-

taminated zone vertically, whereas the 2D ERT defined its

lateral extension along the measured profiles. It is recom-

mended to apply both TEM and 2D ERT techniques at high

density around potential the dump sites to enable a clear

mapping of the direction of the contamination plume.

Demonstration of complete chemical analysis of the col-

lected water samples at Al-Quwy’yia area will be thesubject for future research work.

Acknowledgments This work was supported financially by the

National Plan for Science, Technology and Innovation (NPST) pro-

gram, King Saud University, Saudi Arabia (Project No. 09-ENV836-

02).

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